The present invention relates to a technique effectively applied to a PLL (Phase Locked Loop) circuit in which a VCO (Voltage Controlled Oscillator) is provided and an oscillation frequency can be switched. For example, the present invention relates to a technique effectively used in a radio frequency integrated circuit device provided with a PLL circuit that generates an oscillation signal having a predetermined frequency to be synthesized with a received signal or a transmitted signal in a mobile communication device such as a mobile phone or the like capable of receiving/transmitting signals in multiple bands. Furthermore, the present invention relates to a technique effectively applied to a wireless communication system using this radio frequency integrated circuit device.
According to the examinations made by the inventors of the present invention, with respect to the wireless communication system such as a mobile phone or the like, techniques as shown below are known.
In the wireless communication system such as a mobile phone or the like, a PLL circuit is used as a local oscillator that generates an oscillation signal having a predetermined frequency to be synthesized with a received signal and a transmitted signal. Heretofore, a dual-band mobile phone is known, which can process the signals having two frequency bands, for example, a GSM (Global System for Mobile Communication) having a frequency band of 900 MHz and a DCS (Digital Cellular System) having a frequency band of 1800 MHz. Also, one type of the dual-band mobile phone can handle the two bands by one PLL circuit.